Process for the manufacture of bodies of refractory metals and alloys.



UNITED 1 sTA s PATENT OFFIOE.

PAUL SCHWABZKOPF ANT) SIEGFRIED BURGSTALLER, 0F BERLIN, GERMANY, ASSIGN- OBS TO WOLFRAM LABORATORIUM DRrlNG PAUL SCHWARZKOPF-G. M. B. H., OF

BERLIN, GERMANY.

PROCESS FOR THE MANUFACTURE or BoDIEso ErEAoToRY METALS AND ALLOYS Io Drawing.

To all whom it may concern 7 Be it known that we, Dr. PAUL SOHWARZ- KOPF and Dr. SIEGERIED BURGSTALLER, subjects of the Emperor of Austria-Hungary, residing at Nos. 102/104 Liitzowstrasse,

Berlin, in the Empire of Germany, have invented certain new and useful Improvements in Processes for the Manufacture of Bodies .of Refractory Metals and Alloys, of which the following is a specification.

This invention relates to the production of masses of tungsten, molybdenum and similar diflicultly fusible metals or alloys thereof by sintering processes.

In the manufacture of masses or bodies of tungsten and similar diflicultly fusible metals, it has quite generally been proposed to aggregate the more or less finely divided metal to a coherent mass at a temperature below the fusing point of the particular metal; or in other words to employ a sintering process. Such sintering processes may be divided into two main classes, in the one the metal with a binding agent issquirted into the form of a filament, in the other a relatively massive body of metal is formed which after suitable mechanical treatment is reduced to the desired form; for example, in the manufacture of filaments for electric lamps it is submitted to drawing operations. To this second class of processthe present invention is related.

The initial material to be employed in known processes of this type has been prepared by reducing an oxid of the metal, for example, tungstic oxid, with carbon or hydrogen. At one time, a material essentially consisting of metal in the form of a fine powder was employed as initial material, in the production of which it is stated thatthe linear speed of the moving stream of hydrogen should be great enough to carry away the moisture at a rate which exceeds the rate of diffusion of the watery vapor in the opposite direction. Instead of this finely powdered initial material it has been proposed "to -..employ a coarsely grained metal powder formed by slowly reducing the metal oxid for example, tungstic oxid,

so that the oxid was not reduced immedi- Specification of Letters Patent.

oxid, then to the black oxid, and finally to the pure metal itself. This metal powder is not only coarsely grained but is relatively dense and is especially well reduced, nevertheless-it may contain a certain proportion of unreduced oxid although a less proportion than the more finely grained powders previously employed.

Starting with powders of either of the foregolng types, the sintering process has been carried out by heating a mass pressed up from the powder, in an atmosphere of hydrogen, in order to reduce any oxid which Patented'MaylZ, 1914. Application filed July 5, 1912. Serial No. 707,914.

the powder may by any possibility contain.

The heating has often been effected in two stages, the first causing the material to cohereto some extent, the second at a very much higher temperature, a brilliant white heat, causing the particles to aggregate to a coherent mass which is brittle in the cold.

The object of the present invention is to provide an improved process for the production, by a sintering process, of masses of tungsten, molybdenum and similar diflicultly fusible metals or alloys thereof which shall be capable of being directly mechanically worked.

According to the invention apowder each particle of which essentially consists of metal and oxygen in the same relative proportion is, after consolidation by pressure in molds, subjected to a combined reducing and sintering operation wherein the material is submitted to the reducing action of a reducing gas.

Further, according to the invention; for the purpose of preventing the crystalline structure of the metal body becoming coarse and thus for preventing the development of brittleness,. the initial material employed may contain as an addition, a metal or-a compound of a metal which is not decomposed at the temperature to which the finished body is to be submitted; the said metal or compound being capable of lowering. the vapor pressure .ithout substantially loweringthe melting point of the dilficultly fusible metal forming the main body of the product.

i In the description and claims the sintering process according to the present inven tion will be referred to as a combined sintering and reducing process for the reason that the reduction is a necessary feature of the process; the reducing agent not being employed merely to remove any oxid which may accidentally be present in the mass.

The initial material employed in the present process as previously stated must consist of particles each of which essentlally consists of metal and oxygen in the same relative proportion. .During the combined sintering and reducing operation the pure metal is produced from such material and a coherent mass is obtained. If the initial material contains a high proportion of oxygen a lower temperature will be required in the combined sintering and reducing process than when the oxygen content 1s lower, but, as the removal of oxygen 1s accompanied by a reduction in volume of the material, the product will tend to be more porous the higher the content of oxygen in the initial material and conversely. Consequently it is preferred to employv a material having a low oxygen content and formed by the reduction of an oxid.

In view of the fact that it is necessary to obtain a dense structure in the finished material, the use of a finely grained material suggests itself. It is however necessary to employ material having a high specific gravity and possessing the capacity of being closely consolidated so that the particles after pressing are in intimate contact, and can therefore more easily form crystalline aggregates. Such powder combining a high density and a considerable degree of'fineness has hitherto not been employed, as quite generally it has been found that a powder possesses a lower volume weight the finer it isand conversely. Such preferred form of initial material may be obtained, for example, in the following manner: A higher metal oxid is heated to a relatively high temperature, and a current of hydrogen of high velocity is passed over the heated mass, care being taken in this reduction process to insure that the water vapor is removed as rapidly as it is formed. Thus a very finely grained product is obtained and, in addition, the oxygen partial pressure in the reducing atmosphere is kept so low that reduction directly to metal containing oxygen is effected; in other words the formation of even the lowest oxid of the metal in a stable state is prevented. The high temperature at which ,the reduction is effected insures a product of maximum density being obtained, the high velocity of the reducing gas insures the maximum degree of fineness of grain, a material having the desired proportion of oxygen in the dissolved state being conveniently obtained by stopping the reduction at the corresponding moment; for the production of lower oxids in a stable state is rendered impossible by the said high velocity of the current of reducing gas. This first production may, for example, be

effected in the following manner: Tungstic oxid contained in nickel boats is passed through nickel or iron tubes heated to the required temperature, for example, 1000 C. by means of gas or electricity. The boats are caused by a suitable mechanical device to travel continuously through the heated tube the speed of travel being conveniently maintained at one meter per hour, the reducing gas being passed through in the reverse direction. The tube is provided at each end with chambers which enable boats of'fresh material to be inserted in the one case and permitthe withdrawal of-boats containing the treated material in the other case without interrupting or interfering with the fiow of the gas.

The passage of the hydrogen through the tube at the high velocity desired may be effected by means of a fan or' blower; the velocity being varied in accordance with the results of analysis of the gas leaving the tube. The concentration of water vapor contained in the gas is maintained during the whole process, at least below that corresponding to the known partial pressure of oxygen, at the temperature of the furnace, of the lowest oxid of the metal to be reduced. .The consolidation of the partially reduced powder is effected by known means, strongly reinforced molds of the type in which each surface in succession is at once released from pressure ,being used, in order to preclude the formation of cracks.

The combined reducing and sintering process is best accomplished in a current of reducing gas so that the volatile reaction products are removed continuously. In all cases this process should be-carried out slowly, so that a number of crystal nuclei less than the number of particles of material are produced. The reaction being reversible, the volatile products, if left in 0011- tact with the material, will oppose the reduction and reduce the velocity of the reduction; so also will an oxygen yielding gas or oxygen itself; an addition to the reducing gas of an indifferent gas such as nitrogen will act in a similar manner. In the reduction of tungstic oxid to obtain a metal powder to be subsequently sintered this method of regulating the processof the reduction has been made use of, but in the present instance it is employed in our combined re ducing and sintering process. The progress of the sintering can be thus regulated and the character of the product modified by diluting the reducing gas with an indifferent or an oxygen yielding gas for example steam, or oxygen itself. The progress of this process and the character of the product can also be regulated by regulating the speed of flow of the gas, and the speed at which the volatile products are carried away, as

well as by regulating the composition of the gas. The speed of flow may be expressed by convenient units for example meters per second. A similar result can also be obtained by moving the material to be sintered continuously through a tube, for example, and in the opposite direction to the current of the reducing gas; the speed at which the material is continuously moved in the opposite direction being so regulated that over any cross-section of the tube the composition of the gas and of the material is constant, the reducing atmosphere at the section where the metal is free from oxygen containing the lowest proportion of oxygen; the proportion of oxygen in both continuously increasing toward the end at which the material is introduced. The concentration of the oxygen in the gas phase is always so related to the concentration of oxygen in the solid phase, that the equilibrium is displaced in the direction of the reduction. The velocity of the reaction in addition to being regulated by the speed at which the volatile reaction products are removed may also be increased or diminished by regulating the temperature. The higher the temperature, the rate of the removal of the volatile products being the same, the more rapid will be the reduction. At a higher temperature the crystallization velocity will also be greater. It is thus possible, maintaining a slow speed of reduction and low temperature, to cause only a relatively small number of crystal nuclei to form and to slowly increase in size and then to raise the temperature soas to increase the speed of crystallization and obtain relatively few well developed crystals, or, for example, to carry out the whole process at a relatively slow speed of crystallization, and high rate of reduction, thus obtaining an aggregate of innumerable small crystals.

WVhen the masses of metals are formed according to the present invention, temperatures of 14:50 to 1600 C. suflice for the production of metal which is to be submitted to mechanical treatment of not too severe a type. If however it is to be drawn into thin filaments, for example, into Wire for resistance heaters, filaments for incandescent lamps and the like, after the termination of the reduction process, the material should be submitted to a distinctly higher temperature approaching even the melting pointof the metal. The duration of this treatment need be only a few minutes, and even then, a metal is obtained which has exceptionally valuable mechanical properties; rods having a section of 5 mm. by 5 mm. being capable of being submitted to a drawing process Without previous mechanical treatment such as rolling or hammering. It will thus be observed that the process in which the present invention consists, presents considerable advantages over the hitherto. known processes in that lower temperatures are employed for sintering, the final high temperature treatment, if any, is reduced and mechanical working in order to render the material ductile is despensd with. Further, it is to be observed that by regulating the combined sintering and reducing process any desired character of crystalline metal may be obtained.

In order that the invention may be more clearly understood, the following concrete particulars are given by way of example:

As an initial material a powder each particle of which consists of tungsten and oxygen, has, preferably an oxygen content equal to from say 0.10 to 5 per cent, a content below the lower limit being unsuitable for the present process and a content higher than the superior limit possibly resulting in the exhibition of defects arising from contraction. The preferred size of grain of the initial material lies between 0.l.10- mm. and 0.01.1O mm. The volume of such a powder, determined by introducing l0 grms. thereof into a measuring cylinder and shaking or tappingthe cylinder until the level of the material no longer sinks should preferably be from 1.4 to 2 cubic centimeters; the volume-Weight being thus from 5 to 7 grms. per cubic cm. A powder of this type will be dark gray and will show few, if any, glittering particles to the naked eye. Such an initial material containing for example 0.20% of oxygen is introduced, without a binding agent, into a mold of the. character above referred to and preferably so dimensioned, that the rod obtained will have a length of 15 cms. and a height and width of 5 mms. Larger molds may, with advantage, be employed, for example such as will yield rods 25 cms. in length and 6.5x6.5 mms. in breadth and height. The pressure employed may be 1200 kgs. per sq. cm.; relatively coherent and strong pieces which can be removed from the mold and placed in small tubes or boats without excessive care, being thus obtained. The small tubes or boats employed may be made of Marquardt mass or from a mixture of magnesia and carborundum.

In carrying out the combined reducing and sintering operation, the boats containing the rods are treated in a furnace one form of which is described below. A nickel or steel tube, is surrounded about the middle third of its length by a quartz or porcelain tube, for example, one meter long, without which at the high temperature is capable of sintering to a hard crust, is preferred. The winding-of the resistance wireis so arranged a that the steel or nickel tube is heated so that the temperature gradient from one end on ward over about two-thirds of'tlie length of the quartz tube gradually rises to its maximum value and then gradually falls for the remaining third. The small tubes containing the rods, are inserted into and are caused to uninterrupted-1y travel through the steel or nickel tube; the direction of travel being so arranged that the -boats or small tubes first pass through the region of more slowly increasing temperatures and then through the region where the temperature more rapidly decreases.

The passage of the small tubes through the furnace is effected by a mechanical device, for example they may be pushed through by means of a screw drive, the speed of travel being so regulated that the boats or small tubes move, for example, 5 cms. in four minutes. This speed is however preferably increased if material having an oxygen content approaching the lower limit re-' ferred to above isemployed and is reduced if the oxygen content of the material approaches the superior limit. In the reverse sense to the direction of travel of the boats, a dried, purified stream of hydrogen completely free from oxygen is passed. With a mr al tube of 25 mm. in diameter a gas current flowing at the rate of 300 liters per hour is employed, the linear speed being easily determined from these data making allowance for the-size of the rod and boat. The velocity ofthe gas may be reduced or considerably increased, according to the oxygen content of the initial material, its fineness and density, and the character of procluct required.

When treating an initial material possess ing an oxygen content nearly approaching the lower limit and possessing the maximum degree of fineness and density above referred to, a temperature of 1100 to 1150 0., a rate of travel of the boats in the neighborhood of that given above, and a hydrogen consumption of 300 liters per hour being employed,'rods of an extraordinarily finely crystalline structure are obtained which show that the individual particles constitutingthe initial material have not appreciably increased in size during the sintering process. The rods possess also a very considerable degree of strength and are,'at the conclusion of the sintering process, secured between suitable contacts which may be fixed or movable, they can be in the form of fixed metal clips or in place of one of them a liquid contact may be chosen. The choice depends upon how far the smterlng process in the previously described operation has progressed and Whether as a result a greater or less contraction is to be counted upon. A bell is placed over the contact with the rod in position and a stream of hydrogen is passed through the vessel; the stream being treated and regulated in a similar manner -as in the previously described process. In

this current of gas the rod is gradually heated to incandcscence by means of a current. of electricity. If a finely crystalline product such as that-referred to above is treated, the temperature is raised to 2600 2700 C. and retained constant at this temperature for from 1 to 6 minutes. This temperature lies much below the melting point of tungsten (3300 (1.). The rise 1n temperature to its maximum occupies 15 minutes and the cooling of the rod requires no particular care; The rods thus obtained possess an extraordinarily finely crystalline structure which shows that, during this second stage of the sintering process, no appreciable increase in size of grain results. The rods are moreover, in the truest sense of the word ductile. In the cold the edges can be rounded by means of a hammer and, at a red-heat, a rod can be bent-to a ring, as if it were of steel. These .rods can be treated by the known methodsassociated with-the manufacture of steel, 'for example, rolling, hammering and subsequent drawing; in which however the reduction factor must not be so small a that recommended in the. manufacture of Ste 1. The wire thus obtained. possesses in fact a tensile strength at least equal to that possessed by the best steel wire.

In the preparation of bodies of tungsten, molybdenum or other diflicultly fusible metals or their alloys which should maintain their resistance to action of high temperature, it has been proposed to add diflicultly' fusible metals or compounds thereof which lower the melting point of tungsten or the like metals to an inconsidcrable extent or even, possibly, raise their melting points, such additions remaining in the finished product. The realization of this effect, although apparently this has not been recognized, is in part due, especially if the material added forms a solid solution with the mass forming the body, to the lowering of the vapor pressure of the tungsten or other metal, by which means'the development of larger crystals from the smaller crystals and thus the developmentof a coarse crystalline structure and consequent brittleness is retarded. As, however, in the known processes the whole process for the preparation of the masses of tungsten and the like corresponds to the changes resulting inthe development of brittleness when in use, the use of a material which restrained the development of this undesirable property also hindered the progress of the sintering operation. If, however, these materials are employed in connection with the improved ward at a temperature below, that at which the added material will form a solid solution and therefore they do not in these circumstances effect a lowering of the vapor pressure. It is therefore possible, according to the present process, tochoose the material most suitable for maintaining the properties of the material to be submitted to a high temperature and to determine the most suitable percentage to be employed without having to regard the harmful effect that suchan addition, might have during the process of aggregation by sintering the mass. As particularly sultable additlons may be mentioned uranlum or compounds of uranium which, at the temperature to whichthe finished material is submitted, are subject to no chemical change. For incorporating these additions with the material forming the maif .portion of the mass, any known method may be employed. For exemple, the addition in the form of powder may be mixed with the powdered initial material and the mixture compressed in the manner previously described, the proportion, for example, approaching 2 per cent. In order to obtain a, more intimate mixture, a solution containing a compound of each of the two materials may be taken, the materials precipitated together and the precipitate then worked up to a material of the character described.

e claim 1. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists in first partially reducing their oxids or mixtures of them, compressing the pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen.

2. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists first partially reducing their oxids or mixtures of them, so that each oxid forms a product which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respective metal, compressing the pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen.

' 3. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists infirst partially reducing their oxids or mixtures of them so that each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the res ective metal, compressin the ulverulent- P g P mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen. 4;. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists in, first partially reducing their oxids or mixtures of them so that each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respective metal, and is of a very fine grain, compressing the finely grained pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen.

5. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists in first partially reducing their oxids or mixtures of them so that each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respecoxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respective metal, and which possesses the maximum degree of fineness simultaneously obtainable with the maximum degreeof compactness, compressing the finely gramed and compact pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to ehmmate the remainder of the oxygen.

7. A process for the manufacture of I bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, wh ich may be directly worked without sub ectmg them to a mechanical treatment directed toward reducing the coarseness of structure or grain, which process consists in first partially reducing their oxids or mixtures of them so that each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respective metal, and which possesses the maximum degree of fineness. simultaneously obtainable with the maximum degree of compactness, compressing the finely grained and compact pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen, in such a manner that the fineness of structure of the initial material is, as far as possible,

maintained, regulating the structure of the bodies, by retarding the reduction by adding suitable gas to the reducing agent.

8. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which maybe directly worked Without subjecting them to a mechanical treatment directed toward reducing the coarseness of structure or grain, which process consists in first partially reducing their oxids or mixtures of them so that'each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the owest oxid of the respective metal, and which possesses the maximum degree of fineness simultaneously obtainable with the maximum degree of compactness, compressing the finely grained and compact pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of theoxygen, in such a manner that the fineness of structure of the initial material is, as f'ar as possible, maintained, regulating the structure of the bodies, by imparting relative movement in opposite directions to the same and the reducing agent and by controlling the relative speed of the mass and the agent.

9. A process for the manufacture of bodies of tungsten, molybdenum and similar res fractory metals and alloys thereof, which may be directly worked without subjecting themto a mechanical treatment directed toward reducing the coarseness of structure or grain, which process consists in first partially reducing their oxide or mixtures of them so that each oxid forms a product, each particle of which contains alower proportion of oxygen than corresponds to the lowest oxid of the respective metal, and which possesses the maximum degree of fineness simultaneously obtainable with the maximum degree of compactness, compressing the finely grained and compact pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen, in such a manner that the fineness of structure of the initial material is, as far as possible, maintained, and regulating the structure of the bodies, by controlling the temperature at which the reduction is carried out. I

10. A process for the manufacture ofbodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which may be directly worked without subjecting them to a mechanical treatment directed toward reducing the coarseness of structure or grain, which process consists in first partially reducing their oxids or mixtures of them sothat each oxid forms a product, each particle of which contains a lower proportion of oxygen than corresponds to the lowest oxid of the respective metal, and which.

possesses the maximum degree of fineness simultaneously obtainable with the maximum degree of compactness, compressing the. finely grained and compact pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen, in such a manner that the fineness of structure of the initial material is, as far as possible, maintained, regulating the structure of the bodies, by imparting relative movement in opposite directions to the same and the reducing agent, controlling the relative speed of the mass and the agent, and controlling the temperature at-which the reduction is carried out.

11. A process for the manufacture of bodies of tungsten, molybdenum and similar refractory metals and alloys thereof, which process consists in first partially reducing their oxids or mixtures of same, regulating the structure of the bodies by adding a material or materials which do not substantially affect the fusing point of therefractory metals compressing the pulverulent mass or masses thus obtained into formed pieces, and finally treating the same at a sintering temperature in a reducing atmosphere to eliminate the remainder of the oxygen.

In testimony whereof we afiix our signatures'in presence of two witnesses.

DR. PAUL SCHWARZKOPF. I DR. SIEGFRIED BURGSTALLER.

Witnesses WOLDEMAR HAUPT, HENRY HASPER. 

